A scale or a cutting scrap in a small quantity is conventionally mixed with iron ore fines to serve as a sinter raw material. Along with the recent drastic increase in pig iron production, there is a rapidly increasing demand for sinter as an iron-making raw material, thus requiring installation of large-scale sintering machines. The resultant increase in the consumption of iron sources other than iron ore is leading to the utilization of scales and cutting scraps having a high content of grease deposit in larger quantities. From the point of view of pollution control, on the other hand, it is now a general practice to use an electrostatic precipitator for collecting the dust contained in the exhaust gas from a sintering machine.
More specifically, as shown in FIG. 1, a raw sinter mix is fed from a raw material feeder 4 into a pallet 1 driven by sprocket wheels 2, ignited by an ignition furnace 3, and then, sintered while sucking the exhaust gas through a wind box 5 and a duct 6. The dust contained in said exhaust gas in a large quantity is collected by an electrostatic precipitator 7, and said exhaust gas thus cleaned is discharged through a blower 8.
In this practice, however, if a sinter raw material containing a grease deposit such as a scale or a cutting scrap is mixed in a large quantity into said raw sinter mix, the combustion of said raw sinter mix from its upper portion on the pallet 1 by the ignition furnace 3 would raise the temperature of the middle and the lower portions thereof and cause evaporation of the deposited grease, and the grease vapor would be directed, together with the exhaust gas, into the electrostatic precipitator 7 through the wind box 5 and the duct 6. The dust collected in the electrostatic precipitator 7 has therefore a grease content of from about 0.1% to about 10%. This grease, if deposited in a large quantity on the electrodes of the electrostatic precipitator 7, would be burnt by the spark between the electrodes or by the high temperature exhaust gas, and cause troubles such as a fire in the electrostatic precipitator 7.
Because a rolling mill sludge deposited in a waste water pit of a steel rolling mill has a moisture (water) content of from about 20% to about 30% and a grease content of about 2% to about 3% even after dehydration through a filter press, the following measures are taken for the treatment thereof:
(1) To use said rolling mill sludge for reclamation or to dump the same for ocean disposal; PA1 (2) To utilize said rolling mill sludge as an iron source after removing the grease deposit thereof by a chemical washing (i.e., washing with a chemical); PA1 (3) To utilize said rolling mill sludge as an iron source after removing the grease deposit thereof by combustion.
However, in view of the Fe content of from about 60% to about 70% of said rolling mill sludge, method (1) above is not preferable from the standpoint of the effective use of iron resources. Furthermore, the increasing requirement for pollution control is more and more restricting places for reclamation or dumping of the same, and the disposal of rolling mill scale by this method will be impossible in the near future.
Because of the small particle diameters and the high moisture content of from about 20% to about 30%, even after the dehydration through a filter press, said rolling mill sludge has a high viscosity. Treatment of the rolling mill sludge by method (2) or (3) above, therefore, is not easy. In method (2) above, furthermore, the very difficult separation of the grease limits the use of this method.
Method (3) above requires high equipment costs for a fluidized bed furnace, a rotary kiln, a multi-stage combustion furnace and other facilities, and high operation costs. In any of these furnaces, the treatment of said rolling mill scale at a high temperature of from about 700.degree. to about 800.degree. C causes agglomeration of the rolling mill sludge, thus forming another disadvantage of this method. There are many other problems in this method, such as the necessity to install an electrostatic precipitator for collecting the dust contained in the combustion waste gas.
For these reasons, no satisfactory results are being obtained by any of methods (1) to (3) described above.